Problem: $ A = \left[\begin{array}{rrr}2 & 0 & 0 \\ 2 & 1 & 1 \\ 1 & 1 & 2\end{array}\right]$ What is $ A^{-1}$ ?
Answer: $ = \left[\begin{array}{rrr}\frac{1}{2} & 0 & 0 \\ -\frac{3}{2} & 2 & -1 \\ \frac{1}{2} & -1 & 1\end{array}\right]$